The present invention relates to a synthetic-aperture radar system and a connected operating method for monitoring ground and structure displacements, particularly suitable for emergency conditions.
More specifically, the invention relates to a novel synthetic aperture radar sensor with a ground based platform and having interferometric and polarimetric capabilities as to be easily used on field for acquiring and processing in short time data and for immediately using and displaying the processing results.
This sensor is suitable for monitoring hydrogeological instabilities such as landslides, landslips, avalanches, volcanoes and so on, settlings and yieldings of antropic structures such as dams, bridges, buildings and so on, but it is especially suitable for emergency conditions.
In such conditions, the radar system installation and setting up, the beginning of the measurements and the achievement of early results must be accomplished within few tens of minutes so that the rescue personnel intervention could take place as fastest as possible and, above all, in a top safety condition.
Typical examples of emergency conditions are incipient landslides, man-made structures and buildings instabilities and so on.
As it is known, interferometric radar techniques or methods are conventionally used to recover, for different purposes, radar images positional information.
Regardless of the particular application, the radar interferometry technique is based on quantitative comparison of two images of the same scene acquired from positions and at times that, depending on the particular application, may or may not coincide.
The differential interferometry allows, through the comparison of the phases of the signals belonging to two temporally separated acquisitions, to extract a quantitative information concerning the movement of the various portions of the observed scene, that took place between the two acquisitions.
The movements detected by the mentioned radar system, are the projections of the real displacements along the direction connecting the radar to the moving object that is called LOS (Line Of Sight).
This makes possible to measure displacements of the order of the emitted radiation wavelength, typically between a few centimeters and a few millimeters, with accuracies equal to a fraction of the said wavelength, or rather in the most common applications with precision and accuracy levels between 0.3 and 0.7 mm
One of the main applications of differential interferometry is in the field of environmental risks monitoring and in particular in the monitoring of all those phenomena that cause movements of the observed scenario.
Radar interferometry methods can be carried out either by satellite, airborne or ground based sensors.
For small scale and great precision movements monitoring, the above mentioned satellite and airborne sensors have limitations that can be avoided using ground based sensors.
Ground based sensors retain all the typical advantages of radar devices, such as the remote sensing capabilities and the independence from the illumination conditions, while offering maximum flexibility in terms of revisiting time, acquisition geometry, polarization, selectable frequency of the electromagnetic wave transmitted and easy installation (even within a short notice time), thereby assuring the best fitting to any situation.
Moreover, ground based sensors used in differential interferometry applications provide technical simplifications in repeating the acquisitions from the same position, that is known as the “zero baseline” configuration.
However, in locating a suitable position for the system installation, the main difficulties are found during the installation operations.
The installation of the system must be performed on a basement that will grant stability over time and, when needed, the technical solutions to allow the periodical relocation of the system for any future repetition of the measurements.
A ground based synthetic aperture radar is a sensor system typically consisting of two parts, the first one is mainly mechanical and the second one is mainly constituted by electronic and electromagnetic components.
The mechanical part comprises one or two motor driven linear units, usually having a length of a few meters, with a sliding carriage used to support and displace along the unit the electronic module which is firmly anchored to the carriage.
The electronic module comprises a coherent electromagnetic wave emitter/receiver that uses one or two antennas that are carried along on the carriage as well.
The management of the two main electronic and mechanical modules is usually carried out by an external computer that each time has to be physically connected to the sensor system in order to set the measurement, system operating parameters and starting up the acquisitions.
Each individual measurement is usually carried out by a so-called “Stop & Go” method: the carriage is driven along the linear unit and stopped at a number of discrete steps, spaced at constant distance from one another; for each step a band of electromagnetic waves is emitted and the echo coming from the objects hit by the electromagnetic waves is received.
After receiving the electromagnetic echo the carriage is then moved to the following step along the linear unit where a new measurement cycle is repeated.
As the carriage has covered the overall linear unit length and performed measurements at all the discrete steps, the acquired data are transferred from the measurement system to a external or remote computer that applying dedicated algorithms (called focalizing algorithms) will generate the desired radar images.
Using at least two radar images, from the comparison of their homologous pixels signal phases and performing the difference of these phases a third image, called interferogram, is obtained.
The interferogram allows to derive the displacements of the objects belonging to the observed scenario.
The above process is usually carried out by external or remote computers and requires at least a number of, minutes just for processing a single image.
The time for a full scanning covering all the linear unit length depends on many factors, such as the number of discrete carriage stop positions where the individual measurements are carried out, the mechanical part driving speed and the time required at each step for emitting and receiving the electromagnetic waves.
The main factor to be considered in calculating a full scanning time is the time required to physically displace the carriage hundreds of times to the different constantly spaced discrete steps.
The time required for a complete single scanning is usually of the order of about ten minutes.
Thus, a main disadvantage of these systems is that the minimum time for obtaining two individual images and for processing the related data to achieve an interferogram representing the displacements of the objects belonging to the observed scenario is of the order of tens of minutes.
In fact it is necessary to acquire the raw data for the first image, transfer said raw data to be processed by an external computer, and perform an identical procedure for the second image.
The two images complex data must be further processed to achieve an interferogram and apply algorithms for the removal of variable atmospherics disturbance and noise occurred either in the time interval between the two acquisitions or during the acquisition of a single image.
Therefore, quickly moving objects in the observed scenario such as cars, volcanic lava effusions, harbored ships, sea waves, fast landslips and debris flow and so on, cannot be monitored since this kind of radar systems can only supply indications on the displacements of objects moving just a few millimeters in a time period required to acquire the data needed for a single image creation.
The Italian Patent Application No. FI2001A000064 relates to the above mentioned application field.
Thus, up-to-date ground-based radar sensing assemblies, because of the synthetic aperture technique characteristics they are based upon, and since they operate by an interferometric method, have several main limits hindering their use in many operating applications, said limits include:
1. A very low measurement speed;
2. The systems cannot be easily installed and does not have a suitable operating autonomy;
3. A great difficulty in fitting the system hardware to specific requirements of the phenomena to be monitored;
4. It is very difficult to transport the system to or through scarcely accessible and impassable regions;
5. It is not possible to real time supply data, information and alarms;
6. It is not possible to real time and remotely control and send information without an external PC permanently connected to the system;
7. It is very difficult to provide systems which, upon many mechanical component disassembling, reassembling and mechanical parts replacing operations are still suitable to provide measurements consistent with those acquired before these operations;
8. It is not possible to provide consistent results by a coordinate system different from the intrinsic coordinate frame of the radar system, this latter limitation reduces greatly the system efficiency with respect to other sensing systems, if it is required or desired to compare results with those of other different monitoring systems and it is therefore required a georeferencing of the system results to geographic coordinate systems different from its intrinsic coordinate frame.